1. Field of the Invention
The present invention relates to a semiconductor laser module, and more particularly to a semiconductor laser module having a suitable structure for reducing thermal stress arising from bonding of elements at the time of packaging.
2. Description of the Related Art
In recent years, semiconductor laser elements for mounting on a semiconductor laser module for use in optical communication have come to require higher output and greater accuracy in wavelength. There is a further need to prevent deterioration in element performance in terms of the occurrence of kinks and an increase in current threshold and thereby to improve the production yield. To meet these requirements, it is necessary to reduce thermal stress that occurs when a semiconductor laser element is soldered onto a submount and the base.
In view of this need, there is known a configuration, as described in JP-A-5-299699, in which a submount using a base metal of aluminum nitride (AlN), whose linear expansion coefficient is close to that of the semiconductor laser element, is used over a base of a ferrous material, and the thickness of the submount is properly set to reduce stress.
Also known is a configuration, as described in JP-A-2001-168445, which is designed to reduce stress by properly setting the combined thickness of the semiconductor laser chip and the submount and the width of the semiconductor laser chip when the chip and the submount are to be mounted on a base of a ferrous material (or a copper-based material).
However, the present inventors made research work focusing on the thickness of the submount taken up in JP-A-5-299699 and on the combined thickness of the semiconductor laser chip and the submount taken up in JP-A-2001-168445, and found that stress cannot always be sufficiently reduced merely by adjusting the thickness of the submount. If the stress reduction is insufficient, such defects as current threshold fluctuations or kinks will arise, resulting in a problem of a yield drop in the manufacture of semiconductor laser modules.